Add external links to extension module guide (pypa#446)

This commit adds links to a series of articles hosted at thomasnyberg.com
describing how cpython loads extension modules in more detail.

It includes these as part of a new "Additional Resources" section, so we can readily accommodate other useful links as well.

Author: ApproximateIdentity

source/guides/packaging-binary-extensions.rst

@@ -295,3 +295,23 @@ For interim guidance on this topic, see the discussion in
    mention the fact that Linux distros have a requirement to build from
    source in their own build systems, so binary-only releases are strongly
    discouraged
+
+
+Additional Resources
+====================
+
+Cross-platform development and distribution of extension modules is a complex topic,
+so this guide focuses primarily on providing pointers to various tools that automate
+dealing with the underlying technical challenges. The additional resources in this
+section are instead intended for developers looking to understand more about the
+underlying binary interfaces that those systems rely on at runtime.
+
+Introduction to C/C++ extension modules
+---------------------------------------
+
+For a more in depth explanation of how extension modules are used by CPython on
+a Debian system, see the following articles:
+
+* `What are (c)python extension modules? <https://thomasnyberg.com/what_are_extension_modules.html>`_
+* `Releasing the gil <https://thomasnyberg.com/releasing_the_gil.html>`_
+* `Writing cpython extension modules using C++ <https://thomasnyberg.com/cpp_extension_modules.html>`_